1. Field of the Invention
The present invention relates to a sintered valve seat for automobile engines, and relates to a production method therefor. In particular, the present invention relates to development technique of sintered valve seat which is advantageously used for high load engines, for example, compression natural gas (=CNG) engines and heavy duty diesel engines.
In recent years, operation conditions of automobile engine are very sever due to high performance thereof. Valve seats for engine are required to endure use environments which are severer than those of conventional techniques. For example, in liquefied petroleum gas (=LPG) engine widely used in automobile for taxi, since sliding surfaces of valve and valve seat are used in dry conditions, wear occurs more quickly than that in valve seat for gasoline engine. In environments in which sludge is adhered to valve seat for highly leaded gasoline engine, when surface pressure to valve seat is high or valve seat is used in high temperature and high compression ratio condition, wear is accelerated by the sludge. When valve seat is used in the above severe environments, valve seat is required to have good wear resistance and high strength for prevention of setting.
Valve operating mechanism equipped with a rush adjuster, which can automatically control position of valve and drive timing of valve even when valve seat is worn, has been used. However, engine life problem due to wear of a valve seat is not solved, and development of materials for a valve seat which is superior in wear resistance is desired. In recent years, not only high performance but also development of inexpensive automobile has been important from viewpoint of economic efficiency. Therefore, sintered alloy for a valve seat is required to be highly wear resistant and have high strength without additional mechanism such as the above rush adjuster.
Regarding the above sintered alloy used for valve seat, Japanese Examined Patent Application Publication No. S59-037343 (hereinafter referred to as “Patent Publication 1”) proposes a technique in which Co—Mo—Si based hard particle is dispersed into dapped matrix of Fe—Co based matrix and Fe—Cr based matrix. Japanese Examined Patent Application Publication No. H05-055593 (hereinafter referred to as “Patent Publication 2”) proposes a technique in which Co—Mo—Si based hard particle is dispersed into Fe—Co based matrix. Japanese Examined Patent Application Publication No. H07-098985 (hereinafter referred to as “Patent Publication 3”) proposes a technique in which Co—Mo—Si based hard particle is dispersed into Fe—Co based matrix including Ni. Japanese Unexamined Patent Application Publication No. H02-163351 (hereinafter referred to as “Patent Publication 4”) proposes Fe base alloy including Co—Mo—Si base hard particle dispersed thereinto.
Although the hard particle in each alloy disclosed in Patent Publications 1 to 4 includes 40 mass % or less of Mo, the sintered alloy including the above hard particle is very wear-resistant at high temperatures and has high strength. However, in recent years, sintered alloy is desired to be more wear-resistant at high temperatures and has higher strength. In particular, in engine, for example, CNG engine and high-power heavy duty diesel engine, load to valve seat due to metal contact is much higher, so that development of material which is more wear-resistant in the above environments is desired.